First, consideration as there aren't published B-M's for these "efficient" 9800GT is set them in performance. This might actually be a G94 fab, but I've only heard of 9600GT ECO using those. We might place this similar to the specifications of a mobile 9800GS and that was based on G94 chipsets. This is surely better than a 9600GT with AA at same resolutions, and maybe a little more powerful to get you a few extra FPS at 1920x with a more tiles. A 10% cut in clock doesn't directly equate to a general 10% loss, some titles might not see a full 10% other... it could be more. There's one thing to understand is that Pixel and Texture fill rate are tied to clock speed so those are much reduced (~20%) against a 9600GT.
http://www.gpureview.com/show_cards.php?card1=575&card2=591

So, what newer cards or current offerings would provide low(er) power, better performance; although "no" 6-pin is not truly a hard requirement if the cards' overall has lower power envelope and you have a decent 350W PSU. That field narrows in the $80 deals to 4670, GT240 (GDDR5), 5670, and the 4770.
http://www.techpowerup.com/reviews/MSI/HD_5671GB/1.html

The 4670 - Low power, somewhat lower price, but never going to match in gaming performance. Though you will get a card that at least provides latest Dx10.1 compliance and utilization of 4.1 Pixel/Vertex Shader model.
GT240 (GDDR5) – Good or better on power, probably spars with similar performance in many titles and when the Gigabyte has been $55 with a $25 rebate a good deal. With Dx10.1
5670 – Probably more often on par or better perf